Page 116 - High Power Laser Handbook

86 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s High-Power Fr ee-Electr on Lasers 87

quantum efficiency (up to 15 percent). Even when the cathode is not
poisoned by an imperfect vacuum, however, back bombardment of
ions created by the electron current onto the cathode surface can
result in lifetime limitations. Lifetime is thus governed by total inte-
grated charge delivered, rather than by time.
The laser source for photocathodes can be doubled, tripled, or
quadrupled yttrium aluminum garnet (YAG) or yttrium lithium
fluoride (YLF), depending on the cathode material. A number of dif-
ferent materials have found favor at different institutions: Cs Te has
2
13 percent quantum efficiency (QE) at 263 nm, with lifetimes of
hundreds of hours; LaB , 0.1 percent QE at 355 nm, with lifetimes of
6
24 hours; K CsSb, 8 percent QE at 527 nm, with lifetimes of 4 hours;
2
Cs Sb, 4 percent QE at 527 nm, with lifetimes of 4 hours; and GaAs(Cs),
3
5 percent QE at 527 nm, with lifetimes greater than 40 hours (see Refs. 16
and 17 for a review of many cathode materials). The lifetime data
quoted here should be taken with some degree of skepticism, because
little attempt has been made to unfold the effect of delivered charge
and therefore back bombardment of the cathode life. Some cathode
materials can be rejuvenated many times with oxygen cleaning and
recesiation. Often, injector designs incorporate either a means to pre-
pare and transfer new cathodes to the cavity or a cassette with mul-
tiple cathodes. For high-average current production, the use of UV
laser sources is problematic because of the average power required,
despite the relative robustness of the UV cathode materials. In the
green (doubled YLF), it takes 22.4 W to produce 100 mA from a
1 percent QE cathode. Quadrupled YLF would require 44.8 W of
short-pulse, mode-locked light to produce the same 100 mA at
1 percent QE. Such lasers are well beyond the commercial state of
the art, and lifetime issues associated with the doubling crystals in
the UV are an unsolved problem. Achieving the desired stability in
phase and amplitude, as well as in reliability in the drive laser, is
also not trivial. One would like amplitude stability of 0.5 percent or
better and phase stability between the pulses of less than 1 picosec-
ond (ps). Every doubling multiplies the amplitude noise by two.
To produce higher CW gradients while also delivering excellent
vacuum around the cathode, groups are pursuing the development
of a superconducting RF (SRF) injector cavity. To date, no SRF photo-
gun has been demonstrated beyond some low-current demonstra-
tions; however, such a development would have significant potential
applications. A group at Forschungszentrum Dresden-Rossendorf
who are pursuing such a development believe it is possible to
18
achieve nearly 20 MV/m on the cathode and 10 MV/m average in the
cavity in a tesla-style 3½-cell 1300-MHz cavity. They have constructed
a 1½-cell prototype. Although no fundamental physics issues have
been identified, the engineering challenges are significant. First of all,
it is difficult to hold the cathode accurately in the RF cavity surface
and to prevent RF heating problems that would lead to the cavity